Instructions:

If you have Auto-embeds enabled on your WordPress site, and are trying to embed a Vimeo video by simply pasting in the URL, you may notice that the Embed code comes out as a different size than what is specified in the “Maximum embed size” of your Media Settings.

.Adding a quick filter like this, to your theme’s functions.php file, will adjust the width/height of the auto-embedded iframe from Vimeo, to match the WIDTH of your Maximimim Embed Size.